1. Field of the Invention
The present invention relates to a receiving technology. It particularly relates to a receiving method and a receiving apparatus which controls a weighting coefficient for synthesizing radio signals received by a plurality of antennas.
2. Description of the Related Art
In wireless communication, it is in general desired to effectively use limited frequency resources. In order to use the frequency resources effectively, radio waves of same frequency are, for example, utilized as repeatedly as possible in short-range. In this case, however, communication quality degrades because of cochannel interference caused by a radio base station or mobile terminal closely located, which utilizes the same frequency. As one technology for preventing such communication quality degradation deriving from the cochannel interference, the adaptive array antenna technology can be named.
In the adaptive array antenna technology, signals received by a plurality of antennas are respectively weighted with different weighting coefficients and synthesized. The weighting coefficients are adaptively updated so that an error signal between a signal to be transmitted and the signal after the synthesis might be small. Here, the signal to be transmitted is determined based on the signal after synthesis. In order to update the weighting coefficients adaptively, the RLS (Recursive Least Squares) algorithm, the LMS (Least Mean Squares) algorithm or the like is utilized. The RLS algorithm generally converges at high speed. The RLS algorithm, however, requires a high speed or a huge arithmetic circuit since computation performed is very complicated. The LMS algorithm can be realized with a simpler arithmetic circuit than that of the RLS algorithm. However, the convergence speed thereof is low.
Related Art List
1. Japanese Patent Application Laid-Open No. 2002-26788
In utilizing the adaptive array antenna for a radio mobile terminal, it is suitable to use the LMS algorithm for updating weighting coefficients, since it is desirable that an arithmetic circuit is small. However, the convergence speed of the LMS algorithm is low in general. Thus, if it is desired to delay received signals to be synthesized until the LMS algorithm converges, processing delay becomes large and therefore it is possibly impossible to use the adaptive array antenna in a real time application such as TV conference system where permissible delay time is limited. On the other hand, a response characteristic generally degrades if the weighting coefficients at the timing where the LMS-algorithm has not converged yet in order to diminish the processing delay.